1. Field of the Invention
The present invention relates to a power supply circuit for use in a liquid crystal display apparatus or the like. More particularly, the present invention relates to a high drive performance power supply circuit including a liquid crystal driver, a controller, a memory and the like.
2. Description of the Related Art
According to a conventional technique, there is a series regulator type direct current power supply circuit in which a drive state of a series transistor is controlled, depending on a change in output voltage, so as to suppress overshoot and undershoot during rising of power supply without an increase in capacitance value of an output smoothing capacitor (see U.S. Pat. No. 6,531,855).
According to another conventional technique, there is an operational amplifier in which a phase compensating capacitor and a variable resistance element are connected in series to control the resistance value of the variable resistance element, depending on the magnitude of an input difference voltage, so as to simultaneously achieve a highly stable operation and a high-speed operation (see U.S. Pat. No. 6,137,356).
Of mobile apparatuses, such as, representatively, mobile telephones and the like, apparatuses which have a plurality of functions and include a power supply circuit are becoming more widespread. In such apparatuses, a plurality of power supply voltages required for the functions are generated in the apparatus so that the number of external power supplies to the apparatus is reduced, and power supply is controlled ON/OFF, depending on ON/OFF of the functions, whereby low power consumption can be expected.
In semiconductor integrated circuits, when a power supply voltage is supplied from a power supply circuit to a low voltage transistor block, a regulator is conveniently comprised of an operational amplifier.
When power is externally supplied to a semiconductor integrated circuit, the power supply voltage varies by about 10% to 20%. In this case, in the vicinity of the lower limit of the power supply voltage, the speed is likely to decrease due to the decrease of the power supply voltage. Also, in the vicinity of the upper limit of the power supply voltage, the transistor is likely to be destroyed due to the increase of the power supply voltage. To avoid this, an operational amplifier is used to supply a high-precision power supply voltage. Thereby, a voltage which does not exceed the breakdown voltage of low voltage transistors is supplied, and further, a voltage which does not cause a reduction in speed is supplied, whereby a low voltage transistor block (e.g., a memory) can be comprised of low voltage transistors, resulting in a small area. In addition, the low voltage transistors can have a thin gate oxide film, thereby making it possible to reduce the parasitic capacitance and thereby increasing the speed.
However, the operational amplifier included in the regulator needs to withstand a voltage higher than or equal to the breakdown voltage of the transistors in the low voltage transistor block. For example, in a liquid crystal display apparatus, it is assumed that the breakdown voltages of the controller and the memory are 2 V, the breakdown voltage of the source drivers is 6 V, and the breakdown voltage of the gate drivers is 20 V, where the source drivers and the gate drivers are provided as liquid crystal drivers. In this case, power supply circuits for the respective parts are each comprised of transistors having a breakdown voltage which is higher by 1 to 2 V or by one grade than the breakdown voltage of the corresponding part. In the latter case, the power supply circuits for the controller and the memory are each comprised of 6-V transistors, and the power supply circuit for the source drivers is comprised of 20-V transistors.
Thus, the power supply circuit for use in liquid crystal display apparatuses has significant drawbacks in terms of circuit size and power consumption.
Firstly, when each power supply circuit is comprised of transistors having a breakdown voltage which is higher by 1 to 2 V than that of the corresponding functional circuit, a total of five or six types of transistors having different breakdown voltages are required. In addition to this, capacitors having a breakdown voltage higher by one grade, and in some cases, inductors and resistors, are required. As the number of transistors having different breakdown voltages is increased, the semiconductor process cost increases.
Next, when transistors having breakdown voltages higher by one grade than the respective breakdown voltages are used, the area is increased, resulting in an increase in cost of the semiconductor integrated circuit. In this case, when the 2-V transistor and the 6-V transistor are compared, the gate oxide film thicknesses and the areas of the diffusion portions of the source and the drain are increased by a factor of about 2 to 4. Further, the minimum transistor gate lengths are different by a factor of 2 to 4. Thereby, the area is increased by a factor of 4 to 16. Further, the increase of the gate oxide film thickness leads to an increase in variation of the threshold voltage VT of the transistor, and also, a reduction in speed due to a reduction in drive performance and an increase in parasitic capacitance. Therefore, the characteristics are poor, and the foreseeability of the design is low.
Another problem relates to power consumption. When the controller and the memory consume 10 mA, the power consumption is supposed to be the product with the breakdown voltage of 2 V, i.e., 2 V×10 mA=20 mW. However, when a power supply circuit for supplying 10 mA has a breakdown voltage of 6 V, the power consumption is 6 V×10 mA=60 mW, which is 3 times as high as the required power consumption.